Sir Bradley Wiggins and Team Sky are reportedly being investigated by the UK Anti-Doping Agency (UKAD) regarding the delivery of a medical package to a rider in 2011.
According to the Daily Mail, Wiggins, Team Sky boss Sir Dave Brailsford and their medical team are expected to be questioned by UKAD.
The newspaper states that the probe relates to a Team Sky doctor, Simon Cope, travelling from Geneva to La Toussuire with requested medication on June 12, 2011.
The date coincided with five-time Olympic champion Wiggins winning the seven-stage Dauphine Libere road race, a traditional build-up race to the Tour de France.
It has been claimed that the medical package has been confirmed by British Cycling to have been for a Team Sky rider, but reports suggest it did not contain the substance triamcinolone.
Triamcinolone has been at the centre of controversy with Wiggins in recent weeks, after the Russian-linked Fancy Bears hacking group leaked his confidential World Anti-Doping Agency (WADA) data, along with several other athletes.
Leaks allege that Wiggins received permission to use salbutamol to treat asthma in 2008 before receiving three successive exemptions for corticosteroid triamcinolone acetonide in 2011, 2012 and 2013 due to a pollen allergy.
Wiggins' cases have caused particular concern because his three approvals for triamcinolone acetonide coincided with the Tour de France in 2011 and 2012 and the Giro d'Italia in 2013 - his biggest races of all three seasons.
Dutch time trialler Tom Dumoulin claimed his use of Therapeutic Use Exemptions (TUEs) "stinks" while Prentice Steffen, the doctor at Garmin Slipstream who Wiggins competed for before he joined Team Sky, claimed it "doesn’t look good".
The Daily Mail claim the UKAD probe focuses on allegations surrounding the package, including whether it was requested by Team Sky doctor Richard Freeman for Wiggins, who won the Tour de France the following year.
"UKAD is investigating an allegation of wrongdoing in cycling," a statement said.
"In order to protect the integrity of the investigation we will not comment further."
Wiggins and Team Sky have denied any wrongdoing, with claims their use of substances had been undertaken only to treat legitimate medical conditions.
“Team Sky was contracted by the Daily Mail regarding an allegation of wrongdoing,” a Team Sky statement read.
“We take any issues such as this very seriously and immediately conducted an internal review to establish the facts.
“We informed British Cycling of the allegation and asked them to contact UKAD, who we will continue to liase with.
“Team Sky is committed to clean competition.
“Our position on anti-doping is well known and we 100 per cent stand by that.”
The revelations, however, have raised eyebrows because of Team Sky's strong anti-doping stance in which they claim to be fully transparent.
Brailsford has previously suggested that in future they would be prepared to publicly release all TUEs in a bid to increase transparency.
A British Cycling statement said: "British Cycling can confirm there is an ongoing UKAD investigation, with which we are cooperating fully.
"We are unable to comment further at this stage."
Former Team Sky rider Jonathan Tiernan-Locke, who was sacked in 2014 after being handed a two-year ban for irregularities in his blood passport, has claimed he and his British team-mates were offered the painkiller tramadol ahead of the 2012 Road World Championships.
The painkiller, which is not banned but remains on the WADA monitoring list, has been linked with causing crashes in races.
"There was a time where I rode the World Championships and I was offered tramadol," Tiernan-Locke told the BBC.
"I was not in any pain so I did not need to take it, but it was offered freely around."
British Cycling have rejected Tiernan-Locke’s claim.